decrease number of vertices
Look at your model.

The places where you can only see black are the densest, and that is where you want to focus on reducing the poly count. In your image that would be the front springs, and some of the pipes in the track assembly. All the rest of the model looks fairly low poly and balanced.

I am going to go in an manually reduce the poly count.  Don't worry about selecting all  the edges, it is quick and easy to reduce the poly count in a cylindrical object like a spring.

I will do the spring, but the exact same process works for the pipes.  

Start by selecting every other edge loop around the spring.  


Select the edge rings along the length of the spring. You can do that with CtrlAltRMB 

Run Checker Deselect on that selection. From the 3D view header Select > Checker Deselect. If you have a very dense mesh increase the Skip amount in the operator properties section of the tool shelf.
From the 3D view header Select > Edge Loops.
Press X > Edge Loops to remove all those edges.


Now we must do the loops along the length of the spring. Select this in the exact same way as we did before.
  



An animated gif showing the whole process.